1. Field of the Invention
The present invention generally relates image intensifier optical assemblies and more specifically, to an image intensifier optical objective lens that utilizes right angle bends in its optical path.
2. Description of Prior Art
Image Intensifier night viewing systems have been used by the military since the Korean and Vietnam Wars in the late 1950""s and early 1960""s. Since then, steady advances in technology have resulted in the development of high performance, compact devices which can offer quality imaging approaching 20/30 vision during night conditions under partial moon and starlight. Common to most image intensification systems are three major subassemblies, the objective lens which focuses light from the scene, the intensifier xe2x80x9ctubexe2x80x9d which amplifies the incoming image and outputs a correlated phosphor display, and an eyepiece for viewing by a human operator. Since the intensification process depends upon amplification of light collected from the scene, it is desirable to have an objective lens with as much light collecting ability as possible. This capability is characterized by the F# parameter, which is a lens"" focal length divided by its aperture diameter. The lower the F#, the better the light gathering ability. Typical intensification devices have objective lenses with F#""s ranging from 1.0 to 1.5, the limitation most often being that lenses with lower F#""s are difficult to design and impractical to fabricate. Since most image intensifier sensors are used as either rifle sights or head-mounted goggles, the objective lenses are typically all circularly symmetric and cylindrical in volume.
Recent advances in the coupling of image intensifier tubes with CCD cameras enable the sensor to transmit a video signal to the observer, who may then view the imagery on some other display device. This enables the intensifier sensor to be located in areas such as vehicle turrets, building security outcroppings, and other similar locations where the sensor is not accessible for direct viewing by the human eye. In these cases, the packaging requirements may impose restrictions on the sensor layout.
While the prior art has reported using optical assemblies for image intensifiers none have established a basis for a specific optical assembly that is dedicated to the task of resolving the particular problem at hand. What is needed in this instance is the incorporation of a right-angle bend in the optical path of the objective lens of an image intensifier to facilitate packaging requirements.
It is therefore one object of the invention to provide an objective lens of an image intensifier with a right-angle bend in its optical path.
According to the invention, there is disclosed an objective lens assembly for use in an image intensifier over the 0.6 to 0.9 micron spectral bandwidth. The lens assembly includes an air-spaced doublet on the optical axis for collecting and focusing light entering the assembly. A right angle fold prism after the air-spaced doublet on the optical axis bends the optical axis 90 degrees to a reoriented optical axis. A first lens subassembly after the right angle fold prism on the reoriented optical axis corrects for. The light is then focused by a second lens subassembly after the first lens subassembly on the reoriented optical axis. A lens element after the second lens subassembly on the reoriented optical axis provides image plane field flattening over an intensifier image field format. The lens assembly results in an F# within a range of 1.0 to 1.5 and a field of view within the range of 35 to 45 degrees.
The first lens subassembly which follows the fold prism further includes a positive powered doublet where on of the lens elements maybe filter or clear glass. The second lens subassembly may include a doublet and single lens or a cemented triplet.